A practical Δ-Σ modulator design method based on periodical behavior analysis

A method for digital delta-sigma modulator design for fractional-N frequency synthesis is presented in this paper. The design method is based on the modulator periodical behavior analysis. The modulator is designed to maintain a constant output signal sequence length which can be controlled for all input dc levels. As a result, the quantization noise power is spread over a known, arbitrarily large number of tones within the Nyquist bandwidth. Therefore, the quantization noise power never concentrates into a few dominant spurious tones. The sequence length is controlled by applying predefined initial conditions and scaling modulator buses. Consequently, the spurious-free range can be controlled as a function of the bus width. Known modulator sequence length defines the time required for full modulator and synthesizer characterization.